The present invention relates to recording medium with a lubricant overcoat, particularly, a lubricant overcoat with at least two layers, and a method of making the same.
Most modern information storage systems depend on magnetic recording due to its reliability, low cost, and high storage capacity. The primary elements of a magnetic recording system are the recording medium and the read/write head. Magnetic discs with magnetizable media are used for data storage in almost all computer systems.
FIG. 1 shows the schematic arrangement of a magnetic disc drive 10 using a rotary actuator. A disc or medium 11 is mounted on a spindle 12 and rotated at a predetermined speed. The rotary actuator comprises an arm 15 to which is coupled a suspension 14. A magnetic head 13 is mounted at the distal end of the suspension 14. The magnetic head 13 is brought into contact with the recording/reproduction surface of the disc 11. The rotary actuator could have several suspensions and multiple magnetic heads to allow for simultaneous recording and reproduction on and from both surfaces of each medium. A voice coil motor 19, as a kind of linear motor, is provided to the other end of the arm 15. The arm 15 is swingably supported by ball bearings (not shown) provided at the upper and lower portions of a pivot portion 17.
A conventional longitudinal recording disc medium is depicted in FIG. 2 and typically comprises a non-magnetic substrate 20 having sequentially deposited on each side thereof an under-layer 21, 21xe2x80x2, such as chromium (Cr) or Cr-alloy, a magnetic layer 22, 22xe2x80x2, typically comprising a cobalt (Co)-base alloy, and a protective overcoat 23, 23xe2x80x2, typically containing carbon. Conventional practices also comprise bonding a lubricant topcoat 24, 24xe2x80x2 to the protective overcoat. Under-layer 21, 21xe2x80x2, magnetic layer 22, 22xe2x80x2, and protective overcoat 23, 23xe2x80x2, are typically deposited by sputtering techniques. The Co-base alloy magnetic layer deposited by conventional techniques normally comprises polycrystallites epitaxially grown on the polycrystal Cr or Cr-alloy under-layer.
A conventional longitudinal recording disc medium is prepared by depositing multiple layers of metal films to make a composite film. In sequential order, the multiple layers typically comprise a non-magnetic substrate, a seedlayer, one or more under-layers, a magnetic layer, and a protective carbon layer. Generally, a polycrystalline epitaxially grown cobalt-chromium (CoCr) magnetic layer is deposited on a chromium or chromium-alloy under-layer.
The seed layer, under-layer, and magnetic layer are conventionally sequentially sputter deposited on the substrate in an inert gas atmosphere, such as an atmosphere of pure argon. A conventional carbon overcoat is typically deposited in argon with nitrogen, hydrogen or ethylene. Conventional lubricant topcoats are typically about 20 xc3x85 thick.
Lubricants conventionally employed in manufacturing magnetic recording media typically comprise mixtures of long chain polymers characterized by a wide distribution of molecular weights and include perfluoropolyethers, functionalized perfluoropolyethers, perfluoropolyalkylethers (PFPE), and ftnctionalized PFPE. xe2x80x9cMolecular weightxe2x80x9d is the sum of the atomic weights of all the atoms in a molecule. A xe2x80x9cfunctionalizedxe2x80x9d hydrocarbon, such as functionalized perfluoropolyethers, is a hydrocarbon in which an atom or a group of atoms, acting as a unit, has replaced a hydrogen atom in the hydrocarbon molecule and whose presence imparts characteristic properties to this molecule.
PFPE do not have a flashpoint and they can be vaporized and condensed without excessive thermal degradation and without forming solid breakdown products. The most widely used class of lubricants includes perfluoropolyethers such as AM 2001(copyright), Z-DOL(copyright), Ausimont""s Zdol or Krytox lubricants from DuPont.
There is a demand in computer hard drive industry to develop an areal storage density of 100 Gbits/inch2 and higher in the future. With this high areal density, the flying height between the read-write head and the media has to be minimized. Current magnetic hard disc drives operate with the read-write heads only xcx9c10 nanometers above the disc surface and at rather high speeds, typically a few to a few tens of meters per second. Because the read-write head can contact the disc surface during operation, a thin layer of lubricant overcoat is coated on the disc surface to reduce wear and friction. The overcoat thickness of the hard disc on these future disc-drives is estimated to be less than 3 nm.
With the carbon overcoat as thin as xe2x89xa63 nrm, the overcoat may fail to completely cover the magnetic recording layer. Corrosion of the magnetic layer at those locations where overcoat coverage is imperfect could be a major cause of drive failure. To ensure good corrosion resistance, applicants have found that an overcoat having the ability to minimize or prevent corrosion of the magnetic layer is required and a traditional overcoat material, such as hydrogenated (a-C:H) or nitrogenated (a-C:N) carbon, could be insufficient in protecting the hard disc media or read-write head from corrosion at the thickness level of less than 5 nm.
As hard disc drive technology is pushed to higher and higher recording densities, the industry is faced with an increasingly difficult task of maintaining the tribological robustness of the head-disc interface (HDI). The carbon overcoat and the lubricant film on top of the magnetic recording layer are the main components of a hard disc that control its tribological performance. Tribological performance is characterized in several aspects, including wear durability and corrosion resistance of the media. Higher areal density recording demands reduced head-medium spacing, which in turn requires reduced carbon overcoat thickness and, to a lesser degree, reduced lubricant film thickness. However, if the thickness of the carbon overcoat is about 50 xc3x85 or less, the corrosion protection it provides is greatly compromised. Although there are ways to improve corrosion resistance provided by the carbon overcoat properties, e.g., by increasing its density and changing its electrical conductivity, there is a limit as to how much corrosion resistance a carbon overcoat just a few atomic layers thick can provide.
Wear and friction have been recognized as potential problems for the head/disc interface. One solution for improving the wear resistance of the media is proposed in U.S. Pat. No. 5,674,638 (Grill). Grill suggests using a thick fluorinated diamond-like carbon layer of thickness in the range between 3 nm and 30 nm. Grill uses a thick fluorinated carbon overcoat to improve wear resistance, which generally increases with increased thickness.
One way of improving upon the inherently poor corrosion performance of thin carbon overcoats is to consider the lubricant in combination with the carbon overcoat as a means to maximize corrosion protection. Towards this end, solutions mostly involved the use of a lubricant additive, such as phosphazene or phosfarol, which provide an increase in corrosion protection. These additives react in some fashion with high-energy sites on the carbon surface, where it is believed that corrosion is most likely to initiate. Thus, the blocking of these sites from contact with corrosion sources in the environment, such as moisture and oxygen, provides higher corrosion resistance. However the use of lubricant additives can lead to other problems, such as phase separation of the additive and lubricant, which can degrade the flying behavior of the read/write head and cause the drive to fail.
As the fly height decreases, contact between the media and read/write head will likely increase. As a result, there is a need for a high performance carbon overcoat/lubricant system with enhanced protection characteristics. U.S. Pat. No. 6,110,330 discloses a recording media with a read/write zone and a takeoff/landing zone. Because these two zones are likely to experience different stresses during operation, the recording media is designed to accommodate these different stresses. The media is designed with two different carbon coatings. Each coating is optimized for each zone. Once the carbon coatings are in place a lubricant is added, which is believed to interact differently with each coating zone.
A lubricant overcoat containing two or more lubricants has also been used in an attempt to provide additional protection. U.S. Pat. No. 5,498,457 discloses using a lubricant mixture of two fluoropolyethers. These fluoropolyethers are able to cross-link with another, thus providing a more stable lubricant overcoat with a thickness of over 100 xc3x85. U.S. Pat. No. 5,331,487 discloses a lubricant overcoat containing a functionalized perfluoropolyether bonded to a carbon overcoat, and a mobile, non-bonded lubricant. The non-bonded lubricant can be a perfluoropolyether or any type of lubricant with limited volatility.
Despite these advances proposed in the prior art, there still exists a need for a thin overcoat for a magnetic recording medium that has good corrosion resistance and does not have a tendency to phase separate.
An embodiment of this invention is a magnetic recording medium, comprising a substrate; a magnetic layer on the substrate; a carbon overcoat on the magnetic layer; and a lubricant overcoat on the carbon overcoat, the lubricant overcoat comprising an under-layer which is fully bonded to the carbon overcoat and an over-layer on the under-layer, wherein at least a portion of the over-layer can move on the under-layer. The term xe2x80x9ca portion ofxe2x80x9d is defined herein to include all or part of a layer.
The under-layer comprises a compound selected from the group consisting of functionalized perfluoropolyethers, functionalized perfluoropolyalkylethers, phosphazene and phosfarol. The under-layer could be vapor deposited on a fresh carbon surface. The under-layer could have a thickness from about 5 xc3x85 to about 15 xc3x85. In one embodiment, a component of the under-layer has a molecular weight that is less than a molecular weight of a component of the over-layer.
Preferably, the under-layer comprises a compound comprising a molecular weight selected from the group consisting of less than 3000 atomic mass unit (amu, also known as dalton), of less than 2500 amu, of less than 2000 amu, and of less than 1500 amu. The over-layer could have a thickness from about 5 xc3x85 to about 15 xc3x85. The over-layer could comprise a lubricant or a compound comprising a molecular weight selected from the group consisting of at least 2000 amu, of at least 3000 amu, of at least 4000 amu, and of at least 4500 amu. Preferably, the over-layer comprises a lubricant selected from the group consisting of perfluoropolyethers, functionalized perfluoropolyethers, perfluoropolyalkylethers, and functionalized perfluoropolyalkylethers. The carbon overcoat could comprise a thickness selected from the group consisting of from about 10 xc3x85 to about 80 xc3x85, from about 10 xc3x85 to about 50 xc3x85, and from about 10 xc3x85 to about 40 xc3x85.
Another embodiment of this invention is a method for preventing corrosion of a magnetic layer due to contact start-stop of a recording medium, comprising depositing a carbon overcoat on the magnetic layer and depositing a lubricant overcoat on the carbon overcoat, the lubricant overcoat comprising an under-layer which is fully bonded to the carbon overcoat and an over-layer on the under-layer, wherein at least a portion of the over-layer can move on the under-layer.
Yet another embodiment is a method of making a magnetic recording medium, comprising providing a substrate with a magnetic layer, and a carbon overcoat; depositing a lubricant overcoat on the carbon overcoat, the lubricant overcoat comprising an under-layer which is fully bonded to the carbon overcoat and an over-layer on the under-layer, wherein at least a portion of the over-layer can move on the under-layer. The under-layer could comprise a thickness selected from the group consisting of from about 1 xc3x85 to about 30 xc3x85, from about 5 xc3x85 to about 25 xc3x85, and from about 10 xc3x85 to about 20 xc3x85. The over-layer could comprise a thickness selected from the group consisting of from about 1 xc3x85 to about 30 xc3x85, from about 5 xc3x85 to about 25 xc3x85, and from about 10 xc3x85 to about 20 xc3x85. In a preferred embodiment, the under-layer could be deposited on a fresh carbon surface by a vapor lube process. The over-layer could be deposited by a process selected from the group consisting of dip coating and vapor deposition.
Another embodiment if this invention is a magnetic recording medium, comprising a magnetic material and means for resisting corrosion of the magnetic material. Also, xe2x80x9cmeans for resisting corrosion of the magnetic materialxe2x80x9d refers to a dual layer lubricant comprising a fully bonded under-layer on a surface of a carbon-containing overcoat layer and equivalents thereof. In this invention, a layer is considered to be xe2x80x9cfully bondedxe2x80x9d to a surface if the layer cannot be washed off from the surface with a solvent.